The invention relates to (TiO2, Ta2O5)xe2x80x94La2O3xe2x80x94Al2O3xe2x80x94SiO2 glasses that are characterized by a high strain point, a low coefficient of thermal expansion and a capability of being produced as sheet glass by conventional methods.
Glass substrates for electronic devices have been limited to use and processing temperatures not over about 600-650xc2x0 C. For higher temperatures, the only available transparent materials have been fused silica or a class of glass-ceramics. These materials are difficult, and therefore expensive, to produce.
Fused silica provides a high strain point (typically greater than 1000xc2x0 C.) and excellent thermal stability. However, it is difficult to produce and fabricate. Also, it has a low (5xc3x9710xe2x88x927/xc2x0 C.) coefficient of thermal expansion (CTE) that is not compatible with such electronic materials as silicon.
Transparent, spinel glass-ceramics also provide high strain points (typically greater than 900xc2x0 C.), and provide a better expansion match with silicon (25-40xc3x9710xe2x88x927/xc2x0 C.). However, ceramming these materials adds to their cost. Also, their predecessor glasses tend to be very fluid at their liquidus temperatures. This poses a challenge to forming precision sheet glass; as well as other glass forms.
A need exists, then, for a glass that (1) has a high strain point ( greater than 780xc2x0 C.), (2) does not require costly heat treatments after fabrication, (3) that can be formed at a viscosity greater than 103 poises, and (4) can be melted in a conventional melting unit. In addition, the glass must be transparent to visible radiation and be chemically durable. These several qualities are needed in glasses for production of such varied products as flat panel displays, photovoltaic cells, and tubing and fiber applications that require stability at high temperatures.
Flat panel displays employ sheet glass that necessarily is transparent at visible wavelengths as well as into the ultra violet. It is also necessary that the glass sheet be adapted to production of a silicon layer on the glass surface. Initially, the silicon layer applied was amorphous silicon (a-Si). Fabrication of such devices required temperatures no greater than 350xc2x0 C. Suitable glasses were readily available for use under these conditions.
The evolution from a-Si to poly-Si (polycrystalline silicon) as a coating material has presented a major challenge to use of a glass substrate. Poly-Si coatings require much higher processing temperatures, in the range of 600-1000xc2x0 C.
One available substrate material is fused silica. This material has a high strain point of about 1000xc2x0 C. and excellent thermal stability. However, it has a low CTE that is markedly lower than poly-Si. Furthermore, the ability to fabricate this material is limited, and, at best, very expensive.
Another potential candidate is a family of transparent, spinel glass-ceramics. These materials have the required high strain point of  greater than 780xc2x0 C. They are also reasonably well matched to poly-Si in CTE. However, the additional ceramming process adds significantly to the cost of production. Perhaps more important is the fact that the precursor glasses of these glass-ceramics are very fluid at their liquidus temperatures. This presents a serious challenge to formation of precision sheet glass.
Glasses available from Corning Incorporated under Codes 1737 and 2000 can be used for some low temperature applications on the order of 600-650xc2x0 C. This glass is an aluminosilicate glass that contains a mixture of divalent metal oxides and is essentially free of alkali metal oxides. Even this glass must be subjected to special thermal treatment to avoid shrinkage or compaction during poly-Si deposition.
The efficient production of high quality, poly-Si, thin films requires thermal annealing at temperatures in the 800-900xc2x0 C. range. This higher temperature anneal enables shortening the annealing time. It also results in excellent uniformity at the coating-substrate interface, and more stable performance of a device over time. Except for the fused silica and glass-ceramic substrates mentioned above, there has not been a suitable, substrate material available.
A primary purpose of the present invention is to provide a glass that has properties suited to production of a poly-Si coating on its surface.
Another purpose is to produce a glass having a sufficiently high strain point to permit processing at 800-900xc2x0 C.
A further purpose is to provide a glass that can be melted and formed by conventional procedures employed in producing sheet glass, and that can provide a substrate for application of a high quality, poly-Si film.
A still further purpose is to provide an electronic device, in particular, a flat panel display, embodying a sheet glass substrate, produced in a conventional manner, and having a high-quality, poly-Si, thin film on its surface.
Another purpose is to provide a novel glass family consisting essentially of (TiO2 and/or Ta2O5), La2O3, Al2O3 and SiO2, and optionally containing selected oxides including Y2O3, ZrO2, HfO2, SnO2, GeO2, Ga2O3, Sb2O3, B2O3 and/or P2O5.
The invention resides in part in a family of titania lanthana aluminosilicate glasses having a strain point in excess of 780xc2x0 C., a coefficient of thermal expansion of 20-60xc3x9710xe2x88x927/xc2x0 C., a Young""s modulus greater than 8.28xc3x97104 MPa (12 Mpsi), and a weight loss of less than one mg/cm2 in BHF (buffered HF). Titania, or equivalently tantalum oxide, is an essential constituent which serves to lower the CTE of the glass to a value compatible with poly-silicon. These oxides also act as fluxes, steepen the viscosity curve, and increase strain point.
The invention further resides in an electronic device having a poly-silicon film on a transparent, glass substrate, the substrate being a titania lanthana aluminosilicate glass having a strain point in excess of 780xc2x0 C., a coefficient of thermal expansion of 20-60xc3x9710xe2x88x927/xc2x0 C., a Young""s modulus greater than 8.28xc3x97104 MPa (12 Mpsi), and a loss of less than one mg/cm2 in BHF.